Heat assisted magnetic recording (HAMR) generally refers to the concept of locally heating a recording media to reduce the coercivity of the media so that an applied magnetic writing field can more easily direct the magnetization of the media during the temporary magnetic softening of the media caused by the heat source. The heated area in the storage layer determines the data bit dimension. A tightly confined, high power light spot is used to heat a portion of the recording media to substantially reduce the coercivity of the heated portion. Then the heated portion is subjected to a magnetic field that sets the direction of magnetization of the heated portion. In this manner the coercivity of the media at ambient temperature can be much higher than the coercivity during recording, thereby enabling stability of the recorded bits at much higher storage densities and with much smaller bit cells.
The recording media may be heated using a light beam generated by a laser diode and coupled into the recording head. Because the waveguide structure inside the laser diode is quite different from the guiding structure inside the recording head, challenges exist in establishing an efficient, reliable and low cost design for coupling the output of the laser diode to the recording head.
Similar issues exist in the area of optical communication and information processing, for example with respect to coupling light from a laser diode into a single mode optical fiber or into a channel waveguide in a planar optical circuit. There are two major categories of solutions to the problem: to use a grating or end firing. Canonical linear grating couplers require a collimated incident beam so extra lenses are necessary. The position and direction of the collimating lens need to be controlled precisely as the coupling efficiency is very sensitive to the incident angle. A curved grating can couple light into a planar waveguide from a point source, but the positioning accuracy requirement for the laser diode is not relaxed. Additional problems of regular grating couplers include wavelength sensitivity and tight process tolerance. In the end firing scenario, the lateral alignment accuracy of two waveguides should be a small fraction of the mode width. Since the spot size of a single transverse mode laser diode may be about 1 micrometer wide, positioning the laser diode chip in a large volume structure at sub-micron accuracy is required for high efficiency but difficult to achieve.
It is desirable to launch light into the waveguide with a low cost apparatus, having good alignment tolerances and high light delivery efficiency.